THE TRIBOLOGICAL CHARACTERISTICS OF A HIGH-PURITY ALPHA- ALUMINA SLIDING ON A SIMILAR MATERIAL UNDER UNLUBRICATED CONDITIONS ARE DIVIDED INTO FOUR DISTINCT REGIMES. AT LOW TEMPERATURES, T < 200C, TRIBOCHEMICAL REACTIONS BETWEEN THE ALUMINA SURFACE AND WATER VAPOR IN THE ENVIRONMENT CONTROL THE TRIBOLOGICAL PERFORMANCE. THE COEFFICIENT OF FRICTION IN THIS TEMPERATURE RANGE IS APPROXIMATELY 0.04 AND THE WEAR COEFFICIENT IS LESS THAN 0.000001, INDEPENDENT OF CONTACT LOAD. AT INTERMEDIATE TEMPERATURES, 200C <T <800C, THE WEAR BEHAVIOR DEPENDS ON THE CONTACT LOAD. AT LOW LOADS, WEAR OCCURS BY PLASTIC FLOW AND PLOWING; THE COEFFICIENT OF FRICTION IS APPROXIMATELY 0.60 AND THE WEAR COEFFICIENT IS LESS THAN 0.000001. AT LOADS LARGER THAN A THRESHOLD VALUE, SEVERE WEAR OCCURS BY INTERGRANULAR FRACTURE. THE COEFFICIENT OF FRICTION INCREASES TO 0. 85 AND THE WEAR COEFFICIENT INCREASES TO A VALUE GREATER THAN 0. 0001. AT TEMPERATURES ABOVE 800C, FORMATION OF A SILICON-RICH LAYER ON THE WEAR TRACK BY DIFFUSION AND VISCOUS FLOW OF THE GRAIN- BOUNDARY PHASE REDUCES THE COEFFICIENT OF FRICTION TO 0.40, AND THE WEAR COEFFICIENT IS REDUCED TO A VALUE LESS THAN 0.000001. THE RESULTS OF THE WEAR TESTS AND OBSERVATIONS OF THE FUNDAMENTAL MECHANISMS CONTROLLING THE TRIBOLOGICAL BEHAVIOR OF THIS MATERIAL ARE CONSOLIDATED IN A SIMPLE WEAR TRANSITION DIAGRAM.